/Og (Global Optimizations)

/Og

This option provides local and global optimizations, automatic-register allocation, and loop optimization.

• Local and global common subexpression elimination

  In this optimization, the value of a common subexpression is calculated once. In the following example, if the values of b and c do not change between the three expressions, the compiler can assign the calculation of b + c to a temporary variable, and substitute the variable for b + c:

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  a = b + c;
  d = b + c;
  e = b + c;
  

  For local common subexpression optimization, the compiler examines short sections of code for common subexpressions. For global common subexpression optimization, the compiler searches entire functions for common subexpressions.

• Automatic register allocation

  This optimization allows the compiler to store frequently used variables and subexpressions in registers; the register keyword is ignored.

• Loop optimization

  This optimization removes invariant subexpressions from the body of a loop. An optimal loop contains only expressions whose values change through each execution of the loop. In the following example, the expression x + y does not change in the loop body:

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  i = -100;
  while( i < 0 )
  {
      i += x + y;
  }
  

  After optimization, x + y is calculated once rather than every time the loop is executed:

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  i = -100;
  t = x + y;
  while( i < 0 )
  {
      i += t;
  }
  

  Loop optimization is much more effective when the compiler can assume no aliasing, which you set with Assume No Aliasing (/Oa) or Assume Aliasing Across Function Calls (/Ow).

  Note   You can enable or disable global optimization on a function-by-function basis using the optimize pragma with the g option.

For related information, see Generate Intrinsic Functions (/Oi), Improve Float Consistency (/Op), and Full Optimization (/Ox).

To set this compiler option in the Visual Studio development environment

1. Open the project's Property Pages dialog box. For details, see Setting Visual C++ Project Properties.
2. Click the C/C++ folder.
3. Click the Optimization property page.
4. Modify the Global Optimizations property.

To set this compiler option programmatically

See GlobalOptimizations Property.

See Also

/O Options (Optimize Code) | Compiler Options | Setting Compiler Options